Lazy Function Definition Pattern

gistfile1.js

//Lazy Function Definition Pattern 
// form http://michaux.ca/articles/lazy-function-definition-pattern

var getScrollY = function () {
    if (typeof window.pageYOffset == 'number') {
        getScrollY = function () {
            return window.pageYOffset;
        };
    } else if ((typeof document.compatMode == 'string') && (document.compatMode.indexOf('CSS') >= 0) && (document.documentElement) && (typeof document.documentElement.scrollTop == 'number')) {
        getScrollY = function () {
            return document.documentElement.scrollTop;
        };
    } else if ((document.body) && (typeof document.body.scrollTop == 'number')) {
        getScrollY = function () {
            return document.body.scrollTop;
        }

    } else {
        getScrollY = function () {
            return NaN;
        };

    }
    return getScrollY();
}